The present invention relates generally to ultraviolet-visible spectrophotometers and absorbance detectors, and, more particularly, to a novel method and accompanying apparatus for extending the linear dynamic range of such detectors by separation of incident light before a series of variable path length cells.
A widely used method for monitoring various characteristics of a sample of interest relies upon obtaining accurate measurements of light absorption by the sample. Such measurements are commonly performed as a function of wavelength. For example, the concentration of a solute in a solution can be determined quantitatively by comparing a measured intensity of light transmitted through a sample to a reference light intensity, or alternatively the qualitative identity of the solute may be inferred by considering the various specific wavelengths of light that are absorbed by the sample. In many laboratory and industrial uses of spectrometry the relative intensities or absorbance values at selected wavelengths are needed and the analysis of every point in a complete spectrum is not necessary. Often the point of an analysis is to derive a concentration of one or more components that absorb light. In other cases, points from the absorbance spectrum may be used to define a quality indicator using regression or chemometric techniques. These quantitative analysis applications of spectrometry may require the use of absorbance or intensity values obtained at widely different wavelengths with absorption or emission values that vary substantially. A spectrometer is normally configured to provide the highest accuracy at one wavelength, causing measurements at other wavelengths to be less accurate.
Many spectrometers are available that use a single path length with a single light source and many detectors including photodiode arrays such as Kuderer, U.S. Pat. No. 4,958,928, Kuderer, U.S. Pat. No. 5,116,123, and Bilhorn, U.S. Pat. No. 5,173,748. These systems typically use a monochrometer between the sample and the detector to separate out a single wavelength for measuring absorbance. By scanning the monochrometer, absorbance can be measured at different wavelengths, but not simultaneously. Optical Coating Laboratories, however, has disclosed a miniature spectrometer with optical filters instead of a monochrometer as disclosed in Anthon, U.S. Pat. No. 6,057,925. In Wang, U.S. Pat. No. 5,408,326, a two-wavelength absorbance detector was disclosed that uses two independent light sources and a single sample pathlength to measure two values simultaneously. Additionally, prior art multiple wavelength systems such as the Ocean Optics PC2000 unit or the CVI SM200S unit extract substantial information as a function of wavelength simultaneously, but resolution is limited because intensity information is gathered by 12-bit or 16-bit count CCD elements. CCD based systems inherently have limited dynamic range due to the limits of charge accumulation in the device well and the analogue-to-digital conversion resolution. CCD elements with a 12-bit well depth or a 16-bit well (deep well) depth are available, which provide 4096 and 65536 increments of intensity, respectively. In addition to CCD arrays, diode array instruments can be used, but both generally require significant processor overhead to complete the measurement. Photodiodes or photomultipliers provide much higher, sensitivity, dynamic range and linearity compared to CCD elements. This is discussed in Perkini Elmer Technical Document, Choosing the Detector for your Unique Light Sensing Application, by Larry Godfrey and is available on the World Wide Web at http://opto.perkinelmer.com/library/papers/tp4.htm.
Certainly in industrial measurement and control it is often necessary to reduce a group of measurements at different wavelengths to one or two easy to understand and control process parameters. In fact, other characteristics of the sample of interest may be investigated by performing more complex analyses of the absorbance data. Several patents have addressed methods for extracting performance indicators from absorbance data. For example, Richardson et al., U.S. Pat. No. 5,242,602, teach the use of chemometrics or linear regression techniques with multiple ultraviolet-visible absorbance measurements to derive water treatment performance indicators. International Patent Application WO 96/12183 discloses a method of determining quality parameters and the organic content in pulp and paper mill effluents by applying chemometric methods. In the method disclosed in WO 96/12183 the chemometric algorithms are applied directly to the spectroscopic data. The spectra are subjected to data treatment using values from several discrete wavelengths from each particular spectrum. U.S. Pat. No. 6,023,065, issued to Garver, discloses a method for monitoring and controlling a characteristic of process waters that uses at least two measurements of ultraviolet light absorption to construct a ratio for computing an empirical value of the characteristic of the effluent or process. Garver taught that the use of at least one absorbance ratio to derive a performance indicator improved the information extraction from the absorbance spectrum by providing a means to model non-linear processes and decouple covariant absorbance data. Feedback control is used for adjusting feed input components in accordance with the computed empirical value of the characteristic such that a target measurement of the characteristic is obtained while the excess amount of the input component is kept to a minimum.
According to the method disclosed in U.S. Pat. No. 6,023,065, accurate real-time absorbance data for up to eight different predetermined wavelengths of ultraviolet light are required to obtain empirical values for a plurality of effluent characteristics including: pulp final target brightness; yellowness; residual peroxide; brightness efficiency, yellowness efficiency; and delignification efficiency. For this reason, it will be appreciated that single-wavelength units do not provide sufficient information to determine multiple performance indicators that are dependent on more than one input. Furthermore, the generation of ultraviolet-visible absorbance ratios can multiply signal noise when the absorbance value is in the denominator of the ratio. A very low absorbance at a long wavelength, for example, may be used in a denominator to represent color and an intense absorbance at a short wavelength may be used to represent a bleaching agent such as hydrogen peroxide, for example. In this case the ratio of high absorbance to low absorbance is substantially less accurate than the high absorbance value or the low absorbance value. For example, if the actual ratio is A230/A350, and the error is expressed as err230 and err350 the measured ratio=[A230xc2x1err230]/[A350xc2x1err350]. In a simplified e spectrometer error is 0.01 absorbance units at all wavelengths and absorbance values and absorbance measurements were 1.000 at 230 nm and 0.08 at 350 then the error at 230 nm is 1%. the error at 350 nm is 12.5% and the error in the ratio is xcx9c15%. This simplified example highlights the need for highly accurate absorbance values at different wavelengths when functions with division of absorbance values are used. In practice, different types of accuracy, resolution, and linearity increase error at both high and low absorbance values. An absorbance detection system is typically optimized for measurements between 0.3 and 0.9 absorbance units.
In general, measurements of a quantitative nature entail a prior calibration of the instrument response using at least two different calibration standards of the sample of interest to prepare an absorption curve. Preferably, the prior calibration of the instrument response is such that the light absorption by the fluid sample tends toward an amount of absorption approximately central to an approximately linearly varying region of the absorption curve for the sample at a predetermined wavelength of light. Unfortunately, prior art ultraviolet-visible spectrophotometers are optimised for providing an accurate measurement of light absorption by a sample for only a narrow range of wavelengths of the electromagnetic spectrum. An absorbance measurement using light from other than the optimal wavelength range is obtained with reduced accuracy due to the limited linear dynamic range of the instrument and the decreased digital resolution.
Absorbance is defined as:
A=xe2x88x92log(I/Io)=xcex5xcexclxe2x80x83xe2x80x83(1)
where:
A=the absorbance in absorbance units,
Io=the quantity of incident light provided by the source,
I=the quantity of light transmitted through the sample and to the light detector,
xcex5xcex=the wavelength dependent molar extinction coefficient of the sample,
c=the sample concentration in moles/liter, and
l=the path length of the measurement cell in cm,
wherein the wavelength dependent molar extinction coefficient xcex5xcex of the sample can vary substantially with wavelength. As a result of the wavelength dependence of xcex5xcex, single path-length spectrophotometers lead to a dynamic range problem when performing absorbance measurements at a plurality of different wavelengths. Specifically, the instrument response is calibrated to measure accurately the absorbance of light at a first predetermined wavelength xcex(1) such that the product xcex5xcex(1)cl from Equation (1) yields an absorbance value that is approximately equal to the median value of the highest calibration standard concentration and the lowest calibration standard concentration. Unfortunately, the molar extinction coefficient xcex5xcex(2) of the sample at a second predetermined wavelength of light xcex(2) is likely to be substantially different, and thus the product xcex5xcex(2)cl will correspond more closely to one of the highest calibration standard concentration or the lowest calibration standard concentration. Alternatively, the product xcex5xcex(2)cl is beyond the range of absorbance values for which the instrument response has been calibrated. Measurements of light absorption performed at high absorbance or at low absorbance are more statistically prone to errors and insufficient digital resolution. These arguments may be applied to any measurement of light extinction, not just light absorbance. Light extinction (attenuation) is a complicated function of the light absorption of a liquid; the light absorption of particles, if present; the light emission by fluorescence from dissolved or colloidal components; and the scattering that deflects light away from or towards the detector.
A solution is to vary the path-length through the sample for light of each different predetermined wavelength, to optimize the accuracy of the absorbance measurement at each predetermined wavelength. Variable path-length instruments are known in the art. For instance, LeFebre et al., U.S. Pat. No. 4,786,171, O""Rourke et al., U.S. Pat. No. 5,168,367 and Prather, U.S. Pat. No. 5,268,736. discloses devices that use one of a servomechanism or a linear stepper motor to vary the path length of light through a sample. Unfortunately, these devices are not well suited for obtaining simultaneous absorbance measurements at a plurality of different wavelengths, each measurement requiring a unique path length. The concurrent measurement of light absorption at a plurality of different wavelengths is crucial for on-line analysis of effluents, such as in the pulp and paper manufacturing industry, where process feed back control is required to maintain a desired characteristic of the product stream. In addition, the path length variation of the prior art systems is based upon a mechanical adjustment to the length of the sample cell, which raises a concern about the accuracy and reproducibility of the mechanical mechanism during extended periods of operation.
Xu et al., U.S. Pat. No. 5,602,647 disclosed a different variable path-length instrument that employs a wedge-shaped sample cell having a cross-sectional form of a right-angle triangle. A collimated beam of monochromatic laser light is launched into the sample cell through a light transmissive surface that is normal to the direction of propagation of the light. The light exits through a second light transmissive surface that is equivalent to the hypotenuse of the right-angle triangle. Thus, the light exiting the cell travels a different optical distance in dependence upon the point at which the light originally entered the cell. A multiplicity of photo detectors is arranged parallel to the exit surface of the cell for detecting the intensities of the rays of transmitted light, having traveled different optical path lengths through the sample, at positions of an equal distance from the cell. Alternatively, the sample cell is constructed with a staircase shape such that the light transmissive entrance surface is disposed parallel to a plurality of smaller light transmissive exit surfaces. The optical path length through the sample cell is measured along a line normal to the light transmissive entrance surface and normal to the specific light transmissive exit surface through which the light beam exits. Of course, monochromatic laser light is used and thus obtaining absorbance measurements at a plurality of different predetermined wavelengths involves performing a series of individual absorbance measurements, one absorbance measurement at each different predetermined wavelength. In addition, the use of optical elements for focusing the laser light and for enlarging the diameter of the laser beam larger than that of the original complicates further the design of such an apparatus.
In another embodiment of the invention disclosed in U.S. Pat. No. 5,602,647, a rectangular shaped sample cell with a fixed first light transmissive surface and with a moveable second other light transmissive surface is described. A source projects light toward the sample cell, where the path length of the sample cell may be varied by moving the moveable second other light transmissive surface in a direction parallel to the direction of propagation of the light. It is further disclosed that the source may be one of a tunable laser for providing monochromatic laser light or a lamp for providing polychromatic light. Unfortunately, in a case where a lamp for providing polychromatic light is used, a spectral disk including a plurality of individually selectable different filters, which transmit only light of their corresponding wavelengths, is required. Then, the light derived from the lamp is formed into collimated light by a convex lens, where only light of a selected wavelength is incident upon the cell, which is in a state of only one optical path length. The wavelength resolution of this design is limited by the light filter.
Still another variable path-length instrument is described in U.S. Pat. No. 5,773,828, issued to Akiyama et al. The device comprises a plurality of measuring cells, including a case where the cells are different in length from each other, that communicate sequentially with each other through a communication part to form a single gas path. The arrangement of a plurality of sample cells along a single gas path makes the device well suited for its intended use for the concurrent quantitative analysis of multi-components of a gaseous sample at a high accuracy. Advantageously, each measuring cell provides a fixed optical path length that is appropriate for measuring light absorption by one specific component of the plurality of components of the gaseous sample. It is a drawback of the apparatus that a complicated arrangement of cut-on filters and band-pass filters are used for spectrally separating the infrared radiation provided by the source and for directing the separated light, consisting of relatively broad ranges of wavelengths, along the various optical paths. The use of multiple cut-on filters for wavelength selection is an inefficient process in general. More specifically, the efficiency of band-pass filters in the ultraviolet region is low, typically approximately 12%, and the separation of light into ranges narrower than the order of tens of nanometers is unachievable. In the ultraviolet region of the electromagnetic spectrum xcex5xcex can vary substantially even over a ten to twenty nanometer wavelength range. Hence, the use of cut-on and band-pass filters for separating ultraviolet light is other than a viable option if high accuracy absorbance measurements are desired, and one of skill in the art would not make reference to it.
On-line monitoring of a process effluent, for providing real-time analysis of effluent characteristics to a feed back system controller, is a critical aspect of process control in the environmental monitoring, effluent treatment, food processing, textile and pulp and paper manufacturing. Methods deriving multiple concentrations or performance indicators such as those disclosed in U.S. Pat. Nos. 5,242,602, 5,842,150, 5,641,966, 6,023,065, 5,616,214 require accurate measurements at different wavelengths for optimal information extraction. It is a limitation of the prior art systems that a measurement of an absorbance of substantially monochromatic ultraviolet light must be performed as a series of separate measurements, one measurement required for each different predetermined wavelength of light. Of course, the prior art systems require a finite length of time to obtain such a series of absorbance measurements, said length of time representing an unavoidable delay before an action is taken in response to the changing conditions of the effluent. The ability to make a rapid, accurate measurement is of considerable advantage in short time frame situations. For example, during chromatographic separation of an analyte a single component may pass a detector in less than 1 second. During the initial phase of a rapid chemical reaction, the changes are very rapid and sequential measurements potentially will fail to accurately quantify a changing chemical composition. The inability to rapidly make many measurements that are used for optimization and control may lead to inefficient operation. Potential losses to the company include: lost productivity and high capital costs associated with replacing damaged machinery; lower revenues and higher production costs due to excessive production of off-grade product; and, cleanup costs, fines and poor public image following the release of unacceptably high levels of toxins into the environment.
It would be advantageous to provide an apparatus for obtaining concurrently an accurate, on-line measurement of the quantity of light absorbed by a product stream or effluent, at each of a plurality of different predetermined wavelengths where the illumination and detection at each wavelength is optimized for accuracy.
In order to overcome these and other limitations of the prior art, it is an object of the invention to provide an apparatus for obtaining simultaneously on-line measurement of the quantity of light absorbed by a product stream or effluent at each of a plurality of different predetermined wavelengths.
According to the present invention a portion of fluid matter is diverted through a series of sequentially connected measuring cells, the fluid contained by each measuring cell having substantially a same composition. Each measuring cell is disposed along a separate optical path for measuring the quantity of approximately monochromatic light absorbed by the product stream. The optical path length of each measuring cell is unique, being determined in dependence upon the wavelength of light propagating through each different measuring cell. Further advantageously the configuration of the apparatus supports the use of a plurality of light detectors, one light detector required for each separate optical path. Thus, the wavelength variation of the reference spectrum may be minimized using variable integration times at each independent light detector. The configuration of said apparatus solves several problems related to achieving high accuracy ultraviolet-visible absorption data from a process effluent, which are not adequately addressed by the prior art systems.
In accordance with an embodiment of the current invention, there is provided a method of measuring light absorption by a fluid sample comprising the steps of:
a) providing polychromatic light along an initial optical path;
b) dispersing the polychromatic light in dependence upon wavelength:
to direct light at a first predetermined wavelength along a first optical path having a first path length through the fluid sample; and,
direct light at a second other predetermined wavelength along a second other optical path having a second other path length through the fluid sample;
c) detecting an intensity of light at the first predetermined wavelength after it has propagated the first optical path length through the fluid sample using a first light detector disposed within the first optical path and supporting a first range of detected values; and,
d) detecting an intensity of light at the second other predetermined wavelength after it has propagated the second optical path length through the fluid sample using a second other light detector disposed within the second other optical path and supporting a second range of detected values,
wherein the first path length is selected in dependence upon the first wavelength and the fluid sample such that a detected first value is within a portion of the first range wherein substantial variations in optical intensity result in substantial changes in the first value, and,
wherein the second path length is selected in dependence upon the second other wavelength and the fluid sample such that a detected second value is within a portion of the second range wherein substantial variations in optical intensity result in substantial changes in the second value, and,
wherein the first light detector is angularly disposed along an arc section of a Rowland circle in dependence upon the first wavelength of light and the second other light detector is angularly disposed along a same arc section of a same Rowland circle in dependence upon the second other wavelength of light.
In accordance with another embodiment of the current invention, there is provided a method of measuring light absorption by a fluid sample including a light absorbing species comprising the steps of:
a) providing polychromatic light along an initial optical path;
b) dispersing the polychromatic light in dependence upon wavelength:
to direct light at a first predetermined wavelength along a first optical path having a first path length through the fluid sample; and,
to direct light at a second other predetermined wavelength along a second other optical path having a second other path length through the fluid sample;
c) detecting an intensity of light at the first predetermined wavelength after it has propagated the first optical path length through the fluid sample using a first light detector disposed within the first optical path and supporting a first range of detected values; and,
d) detecting an intensity of light at the second other predetermined wavelength after it has propagated the second optical path length through the fluid sample using a second other light detector disposed within the second other optical path and supporting a second other range of detected values,
wherein the first path length is selected in dependence upon the first wavelength and the fluid sample such that a detected first value is within a portion of the first range wherein substantial variations in optical intensity result in substantial changes in the first value, and,
wherein the second path length is selected in dependence upon the second other wavelength and the fluid sample such that a detected second value is within a portion of the second range wherein substantial variations in optical intensity result in substantial changes in the second value, and,
In accordance with yet another embodiment of the current invention, there is provided an apparatus for measuring light absorption by a fluid sample comprising:
at least a light source for providing polychromatic light along an initial optical path;
a light separating element disposed within the initial optical path for receiving the polychromatic light from the at least a light source and for dispersing the polychromatic light in dependence upon wavelength to direct light at each of a plurality of different predetermined wavelengths along one of a plurality of different secondary optical paths, including a signal at a first predetermined wavelength of light propagating along a first secondary optical path and a signal at a second other predetermined wavelength of light propagating along a second other secondary optical path;
a first channel detector disposed within the first secondary optical path comprising:
a) a first sample cell for containing a fluid sample within a containing portion thereof and having at least a light transmissive endface; and,
b) a first light detector disposed for receiving light at the first predetermined wavelength from one of the at least a light transmissive endface of the first sample cell, light at the first predetermined wavelength propagating a first optical path length through the containing portion of the first sample cell;
a second channel detector disposed within the second other secondary optical path comprising:
a) a second other sample cell for containing a fluid sample within a containing portion thereof and having at least a light transmissive endface; and,
b) a second other light detector disposed for receiving light from one of the at least a light transmissive endface of the second other sample cell, light at the second other predetermined wavelength propagating a second different optical path length through the containing portion of the second other sample cell;
wherein the light separating element defines a Rowland circle, the first channel detector and the second other channel detector being angularly disposed along an arc section of the Rowland circle in dependence upon the first predetermined wavelength of light and the second other predetermined wavelength of light, respectively.
In accordance with yet another embodiment of the current invention, there is provided an apparatus for measuring light absorption by a fluid sample comprising:
at least a light source for providing polychromatic light along an initial optical path;
a dispersive element disposed within the initial optical path for receiving the polychromatic light from the at least a light source and for dispersing the polychromatic light in dependence upon wavelength to direct light at each of a plurality of different predetermined wavelengths along one of a plurality of different secondary optical paths, including a signal at a first predetermined wavelength of light propagating along a first secondary optical path and a signal at a second other predetermined wavelength of light propagating along a second other secondary optical path;
a same sample cell disposed within the first secondary optical path and the second other secondary optical path for containing a same fluid sample within a same containing portion thereof and having at least a light transmissive endface, the at least a sample cell being shaped such that:
light at the first predetermined wavelength propagates along a first optical path length through the same containing portion of the same sample cell; and,
light at the second other predetermined wavelength propagates along a second other optical path length through the same containing portion of the same sample cell,
a first light detector disposed for receiving light at the first predetermined wavelength from one of the at least a light transmissive endface of the same sample cell; and,
a second other light detector disposed for receiving light at the second other predetermined wavelength from one of the at least a light transmissive endface of the same sample cell,
wherein the dispersive element defines a Rowland circle, the first light detector and the second other light detector being angularly disposed along an arc section of the Rowland circle in dependence upon the first predetermined wavelength of light and the second other predetermined wavelength.
Although many prior art spectrometer designs are known, a system including a plurality of sample cells, each sample cell for providing a different optical pathlength through a substantially same fluid sample, and wherein each sample cell is disposed within a different optical path so as to receive substantially monochromatic light at one of a plurality different predetermined wavelengths, and wherein further there is provided within each optical path an individual light detector, the light detector being optimised for measuring a light absorbance by the fluid sample at the wavelength of light that is directed along that optical path, is unknown.